According to a group of researchers in the USA, the energy stored in thermochemical materials can effectively heat indoor spaces, achieving excellent performance, particularly in humid regions.
What are thermochemical materials?
Thermochemical materials (TCMs) are a diverse class of compounds capable of storing and releasing heat through reversible internal chemical reactions. This group includes various hydrated salts as well as silica gel- or zeolite-based adsorbents.
Compared to sensible heat storage and latent heat storage, thermochemical storage theoretically offers higher energy density (100–1000 kWh/m³) and minimal long-term losses due to its independence from temperature. However, real-life applications remain significantly limited.
A new opportunity for TCM applications comes from research conducted by scientists at the National Renewable Energy Laboratory (NREL) of the U.S. Department of Energy and the Lawrence Berkeley National Laboratory. The team studied a realistic configuration to integrate thermochemical materials into heating, ventilation, and air conditioning (HVAC) systems for homes.
To understand the scope of this study, it is essential to make a few clarifications.
Open and closed TCM systems
TCMs owe their thermal storage capabilities to water. By hydrating or dehydrating them—creating or breaking bonds with water vapor—they release or store energy. Systems based on thermochemical materials can be classified as open-cycle or closed-cycle systems, depending on their design and configuration.
Open systems, as the name suggests, are exposed to ambient conditions, with water vapor coming directly from the surrounding air. In contrast, closed systems use an isolated chamber without air, where water vapor is derived from the evaporation of liquid water in a separate chamber.
Open-cycle thermochemical systems are simpler but present challenges during winter. Why? Because using indoor air to drive the hydration reaction can reduce a building’s humidity to an uncomfortable level, while cold outdoor air contains limited moisture.
Thermochemical materials integrated into domestic heating
“The way we integrated the TCM system into the building allowed us to do so without drying out the house,” said Jason Woods, an engineer at NREL and co-author of the new paper on the subject. “It’s important to consider the source of moisture, as performance can be significantly influenced by how it is integrated.”
The team closely studied the thermal performance of a thermochemical reactor powered by a specific salt hydrate. They focused on strontium chloride (SrCl₂), one of the most promising candidates for low- and medium-temperature thermal storage in buildings due to its ease of hydration via water vapor at 70°–150°C.
Like other TCMs, this material emits heat by reacting with water vapor in the air. For their research, the scientists considered different climates, examined various configurations, and paid particular attention to the source of water vapor, modeling the systems and validating the results in real environments.
The best-performing configuration, according to an NREL press release, “allowed the TCM reactor to heat exhaust air from the building, which has the same temperature and humidity as indoor air. Once heated, this air indirectly warms incoming ventilation air through a heat exchanger. This prevents the reactor from dehumidifying the indoor air and ensures sufficient humidity levels.”
How much would such an application cost?
Note: this application works only for buildings where the exhaust air vent is located near the incoming ventilation system. Woods stated that the reactor is not intended to replace a heat pump or boiler but rather to store energy for later use.
Relative humidity proved to be a key factor in the system’s performance. The researchers modeled their reactor for a single-family home, a small hotel lobby, a medium-sized office building, and hospital rooms. The marginal capital cost of a thermochemical storage system decreases as building size increases, with a levelized cost of storage (LCOS) estimated at less than 10 cents per kilowatt-hour.
Read the research in the scientific journal Applied Energy.
